The present invention relates to a continuously variable speed-changing device and, more particularly, to a device for changing the speeds of the front and rear wheels in a four-wheel-drive vehicle, capable of variably driving the front wheels and the rear wheels independently from each other by using a toroidal type continuously variable speed-changing device or a belt-type continuously variable speed-changing device.
There has heretofore been proposed a four-wheel-drive vehicle equipped with a continuously variable speed-changing mechanism for the front wheels and with a continuously variable speed-changing mechanism for the rear wheels in order to variably drive the front wheel drive shafts and the rear wheel drive shafts independently from each other via the continuously variable speed-changing mechanisms, in an attempt to decrease the weight of the four-wheel-drive mechanisms, to avoid the loss of power and to improve the fuel efficiency.
A typical example of the continuously variable speed-changing device in the four-wheel-drive vehicle can be typified by a toroidal type continuously variable speed-changing device disclosed in Japanese Laid-open Patent Publication (Kokai) No. 157151/1993 (JP-A 5-157151). This continuously variable speed-changing device includes an input shaft supported by a casing, and a continuously variable speed-changing mechanism for the front wheels and a continuously variable speed-changing mechanism for the rear wheels that are arranged, spaced apart, in the axial direction of the input shaft. The continuously variable speed-changing mechanism for the front wheels changes the input rotational speed of the input shaft and outputs it to the front wheel drive shafts. The continuously variable speed-changing mechanism for the rear wheels changes the input rotational speed of the input shaft and outputs it to the rear wheel drive shafts. These continuously variable speed-changing mechanisms have been so constituted as to variably drive the front wheels and the rear wheels independently from each other.
The above continuously variable speed-changing mechanism for the front wheels has substantially the same constitution as that of the continuously variable speed-changing mechanism for the rear wheels. Therefore, the constitution of the continuously variable speed-changing mechanism for the front wheels will now be briefly described. The continuously variable speed-changing mechanism for the front wheels includes an input disk which is secured to the input shaft and has one surface in the axial direction thereof formed as an input side recessed surface of an arcuate shape in cross section, and an output disk having one surface in the axial direction thereof formed as an output side recessed surface of an arcuate shape in cross section. The output disk is so disposed as to rotate relative to the input shaft and to surround the outer peripheral surface of the input shaft and has its output side recessed surface disposed so as to be opposed to the input side recessed surface of the input disk in the axial direction. A pair of power rollers are disposed between the input side recessed surface of the input disk and the output side recessed surface of the output disk so as to rotate relative to the input disk and the output disk. The power rollers are supported by trunnions so as to rotate. The axes of rotation of the power rollers are arranged at right angles with the axes of the trunnions that will be described later.
The trunnions are so supported as to rotate about their axes relative to the casing and to move in the axial direction. The axes of the trunnions extend in parallel with each other in the tangential direction relative to the input shaft at symmetrical positions with the input shaft interposed therebetween at an equal distance. On the cross sections of the input disk and of the output disk opposed to each other in the axial direction, the center of arc of the input side recessed surface of the input disk and the center of arc of the output side recessed surface of the output disk are arranged on a common center of arc. Further, the axial centers of the trunnions are arranged to be in agreement with the centers of the corresponding arcs. The axes of the trunnions define tilting axes along which the corresponding power rollers rotate. The power rollers are so constituted as to possess spherically protruded surfaces in the peripheries thereof, the protruded surfaces being brought into a pressed contact (point contact) with the input side recessed surface of the input disk and with the output side recessed surface of the output disk. The trunnions are moved in the axial directions but in the opposite directions relative to each other by the actuators such as hydraulic cylinders, whereby the power rollers rotate about the axes which are tilted by an angle corresponding to the amount of motion of the trunnions and changing of the speed between the input disk and the output disk is performed. According to the toroidal type continuously variable speed-changing device as described above, the rotational forces of the output disks of the continuously variable speed-changing mechanism for the front wheels and of the continuously variable speed-changing mechanism for the rear wheels are output to the front wheels and to the rear wheels independently from each other, to execute the full-time four-wheel drive. Constitutions of trunnions of the toroidal type continuously variable speed-changing device have been disclosed in, for example, Japanese Utility Model Publication (Kokoku) No. 11425/1994 and Japanese Laid-open Patent Publication (Kokai) No. 269039/1997 (JP-A 9-269039).
In the continuously variable speed-changing device, when a steering angle is given to the front wheels, the tilting angle of the power rollers of the continuously variable speed-changing mechanism for the front wheels is differed from the tilting angle of the power rollers of the continuously variable speed-changing mechanism for the rear wheels to drive the four wheels with little loss of power while absorbing the difference in rotational speed between the front wheels and the rear wheels when the vehicle turns, without using the center differential.
Another typical example of the continuously variable speed-changing device can be represented by a belt-type continuously variable speed-changing device. This continuously variable speed-changing device employs a belt-type continuously variable speed-changing mechanism for the front wheels and a belt-type continuously variable speed-changing mechanism for the rear wheels instead of the above-mentioned toroidal type continuously variable speed-changing mechanism for the front wheels and the toroidal type continuously variable speed-changing mechanism for the rear wheels. Each of the belt-type continuously variable speed-changing mechanism for the front wheels and the belt-type continuously variable speed-changing mechanism for the rear wheels comprises a primary pulley having a pulley fixed to the primary shaft drive-coupled to the output shaft of the engine and a movable pulley which is opposed to the fixed pulley and can move in the axial direction, a secondary pulley having a pulley fixed to the secondary shaft and a movable pulley which is opposed to the fixed pulley and can move in the axial direction, and a drive belt wrapped round the primary pulley and the secondary pulley. Upon changing the gaps of the pulley grooves of the primary pulley and of the secondary pulley, the belt-wrapping diameters of the pulleys change and the speed-changing ratio between the primary shaft and the secondary shaft is continuously adjusted. Even by using the thus constituted belt-type continuously variable speed-changing device, the front wheels and the rear wheels can be variably driven independently from each other by the continuously variable speed-changing mechanism for the front wheels and by the continuously variable speed-changing mechanism for the rear wheels in substantially the same manner as in the above-mentioned toroidal type continuously variable speed-changing device.
In the four-wheel-drive vehicle, part of the tire diameters of the wheels often changes not only when a temporary tire is used but also when the tire pneumatic pressure is decreased, the tire pneumatic pressures have not been well adjusted and tread surfaces are worn out. A change in the tire diameter of either the right wheel or the left wheel appears as a change in the average tire diameter of the right and left wheels for the front wheel drive shafts or for the rear wheel drive shafts, since differentials are provided for the front wheel drive shafts and for the rear wheel drive shafts, respectively. When a difference occurs in the tire diameter between the front wheels and the rear wheels, on the other hand, there results inconvenience such as loss of power due to a difference in the rotational speed between the front and rear drive systems, impairing smooth four-wheel-drive traveling. Therefore, this inconvenience must be avoided.
In the case where a difference occurs in the tire diameter between the front wheels and the rear wheels as described above, if the speed-changing ratio for the front and rear wheels is so set that the front wheel drive shafts and the rear wheel drive shafts run at an equal rotational speed, unequal tire diameters of the front and rear wheels turn out to be a difference in the peripheral velocity of the front and rear wheel tires when traveling straight, causing inconvenience such as loss of power. When the tire diameters of the front and rear wheels are not the same, therefore, it is desired to avoid inconvenience such as loss of power ascribed to unequal tire diameters by changing the speed-changing ratios for the wheels so that all of the wheels have an equal peripheral velocity, by utilizing the fact that the speed changing ratios for the front and rear wheels can be independently controlled. The conventional toroidal type continuously variable speed-changing device described earlier is so constructed to allow to carry out the four-wheel-drive with little loss of power while absorbing the difference in the peripheral speed between the front wheels and the rear wheels during turning without using the center differential by imparting a difference to the speed between the front wheels and the rear wheels when the vehicle turns. However, no means has yet been proposed for solving the problem for the case where a difference exists in the tire diameters between the front wheels and the rear wheels. Inconvenience such as loss of power due to unequal tire diameters of the front and rear wheels is possible to occur not only in the above-mentioned conventional toroidal type continuously variable speed-changing device but also in the above-mentioned belt-type continuously variable speed-changing device. Therefore, it is necessary to provide a means for solving the problem.
It is an object of the present invention to provide a novel device for changing the speeds of the front and rear wheels in a four-wheel-drive vehicle, which prevents the occurrence of inconvenience such as loss of power caused by a difference in the tire diameters between the front wheels and the rear wheels and ensures smooth four-wheel-drive traveling.
Another object of the present invention is to provide a novel device for changing the speeds of the front and rear wheels in a four-wheel-drive vehicle, which avoids the inconvenience such as loss of power and ensures smooth four-wheel-drive traveling, by detecting changes in the tire diameters of the front wheels and the rear wheels during traveling straight and, when differences in the tire diameters of the front and rear wheels are detected, separately controlling the speed-changing ratios for the front and rear wheels according to the differences such that the front and rear wheels will assume the same peripheral velocity.
A further object of the present invention is to provide a novel device for changing the speeds of the front and rear wheels in a four-wheel-drive vehicle, which avoids inconvenience such as loss of power caused by a difference in the tire diameters between the front wheels and the rear wheels and ensures smooth four-wheel-drive traveling, by effecting a relatively simple control operation.
According to the present invention, there is provided a device for changing the speeds of the front and rear wheels in a four-wheel-drive vehicle, which is equipped with a continuously variable speed-changing mechanism for the front wheels which changes the input rotational speed and outputs it to the front wheel drive shafts and a continuously variable speed-changing mechanism for the rear wheels which changes the input rotational speed and outputs it to the rear wheel drive shafts, so that the front wheels and the rear wheels can be driven at continuously variable speeds independently of each other owing to the continuously variable speed-changing mechanisms, wherein:
the device for changing the speeds of the front and rear wheels comprising a detector means for detecting the operation conditions of the vehicle and a controller for variably controlling the speeds of said continuously variable speed-changing mechanism for the front wheels and of said continuously variable speed-changing mechanism for the rear wheels based on the operation conditions of the vehicle detected by said detector means; and
the controller calculates a tire diameter ratio of the front wheels to the rear wheels during the vehicle travels straight, sets a target speed-changing ratio obtained based on the vehicle operation conditions detected by the detector means to be a target speed-changing ratio for the rear wheels in the continuously variable speed-changing mechanism for the rear wheels or to be a target speed-changing ratio for the front wheels in the continuously variable speed-changing mechanism for the front wheels, and sets a speed-changing ratio obtained by correcting the target speed-changing ratio based on the tire diameter ratio to be a target speed-changing ratio for the front wheels or to be a target speed-changing ratio for the rear wheels.
It is desired that a driving force disconnection device is provided to disconnect the driving force transmitted to the front wheels from the continuously variable speed-changing mechanism for the front wheels or to disconnect the driving force transmitted to the rear wheels from the continuously variable speed-changing mechanism for the rear wheels, and the controller calculates the tire diameter ratio in a state where the driving force disconnection device is disconnected.
It is desired that the controller calculates the tire diameter ratio from a ratio of the rotational speed of the front wheel drive shafts to the rotational speed of the rear wheel drive shafts.
It is desired that the controller sets the speed-changing ratio obtained by correcting the target speed-changing ratio according to a predetermined equation by using the target speed-changing ratio and the tire diameter ratio as variables to be a target speed-changing ratio for the front wheels or a target speed-changing ratio for the rear wheels.
It is desired that the controller obtains the target speed-changing ratio for the front wheels or the target speed-changing ratio for the rear wheels by adding an amount for correcting the speed-changing ratio calculated based on the tire diameter ratio to the target speed-changing ratio.
It is desired that the controller calculates the amount for correcting the speed-changing ratio in compliance with a predetermined equation or an approximate equation using the tire diameter ratio as a variable.
It is desired that the controller obtains the amount for correcting the speed-changing ratio from a map prepared in advance based on the values calculated or approximated by a predetermined equation or an approximate equation determined in advance by using the tire diameter ratio as a variable.
It is desired that the controller obtains an approximate amount for correcting the speed-changing ratio calculated by a predetermined approximate equation using the tire diameter ratio as a variable, from a map prepared in advance based on the approximate values which have been further corrected based on a deviation of the ratio between the target speed-changing ratio for the front wheels and the target speed-changing ratio for the rear wheels at said tire diameter ratio.
It is desired that each of the continuously variable speed-changing mechanism for the front wheels and the continuously variable speed-changing mechanism for the rear wheels is constituted by a toroidal type continuously variable speed-changing device comprising an input disk for receiving a driving force, an output disk that is disposed opposite to the input disk and is drive-coupled to the front wheel drive shafts or to the rear wheel drive shafts, and power rollers disposed between the input disk and the output disk so as to rotate in a tilted manner and to change contact points to the input disk and to the output disk to continuously change the rotational speed of the input disk and to transmit the rotation to the output disk.
It is desired that the controller sets a target tilting angle of the power rollers for the front wheels in the continuously variable speed-changing mechanism for the front wheels and a target tilting angle of the power rollers for the rear wheels in the continuously variable speed-changing mechanism for the rear wheels, respectively, correspondingly to the target speed-changing ratio for the front wheels and the target speed-changing ratio for the rear wheels, to control the continuously variable speed-changing mechanism for the front wheels and the continuously variable speed-changing mechanism for the rear wheels based on the target tilting angle for the front wheels and the target tilting angle for the rear wheels.